1. Field of the Invention
The present invention relates to an apparatus for optically detecting the position of an opaque mark formed on a transparent substrate. This position detecting apparatus is applicable as a mask alignment apparatus for a projection exposure apparatus (stepper) used in the manufacture of semiconductor devices.
2. Description of the Prior Art
With the recent remarkable improvement in the level of integration of devices such as memory devices in large scale integrated circuits (LSIs), the design line widths of circuit patterns on such devices have reached the levels ranging from 1 .mu.m to 0.5 .mu.m and reduction projection-type projection exposure apparatus (steppers) employing ultraviolet light of a particular short wavelength as its exposure light have been used solely for the purpose. The projection exposure apparatus is such that a mask (reticle) containing a circuit pattern and an exposure specimen (wafer) coated with a light-sensitive material are arranged to oppose each other through a projection optical system whereby after they have been two-dimensionally aligned in a conjugate plane of the projection optical system with respect to the exposure light, the exposure light is irradiated from the backside of the mask and the circuit pattern is photographically transferred onto the exposure specimen.
It is to be noted that a large number of exposure areas of the same pattern are arranged on the ordinary exposure specimen (wafer) whereby the exposure specimen is moved (stepped) and the transfer of the circuit pattern is repeated for every exposure or shot area, thereby completing the processing of the single exposure specimen. Such exposure transfer is repeated a number of times throughout the LSI manufacturing process. As a result, with the projection exposure apparatus employed in the LSI manufacturing process, on one hand, the accuracy of as high as less than 10% of the design line width or 0.05 .mu.m is required for the mask-to-exposure specimen alignment accuracy (the alignment accuracy for the transferred image of the circuit pattern and the exposure area) for every exposure transfer and, on the other hand, it is required that the required alignment time per exposure specimen is reduced so as to improve the efficiency as the manufacturing apparatus. To improve the accuracy and efficiency concerning the mask-to-exposure specimen alignment is one of the important technical problems encountered in the projection exposure apparatus.
As the projection exposure apparatus of the type which simultaneously satisfies the desired alignment accuracy and efficiency, an apparatus of the type in which a mask and an exposure specimen are separately aligned with respect to a projection optical system (or the projection exposure apparatus) has been put in practical use. In this apparatus, a mask is mounted on an X-Y stage provided with a precision measuring device (interferometer whereby while illuminating position detecting opaque marks formed on a transparent mask substrate and detecting detection images due to the reflected light from the marks by photoelectric detectors thereby checking the positions of the marks, the mask is moved by the X-Y stage and the mask is positioned with respect to the projection optical system (or the projection exposure apparatus). Then, an X-Y stage for carrying the exposure specimen thereon is positioned with respect to the projection optical system (or the projection exposure apparatus) by use of a position detecting system for the X-Y stage and a position detecting system for the exposure areas on the exposure specimen. Then, according to this arrangement, the respective exposure areas are successively fed mechanically to the position of the transferred image of the circuit pattern.
At this time, the position detecting marks formed on the mask are usually formed, along with the circuit pattern, by selectively removing a thin film of opaque chromium formed on a transparent glass substrate by etching. There are instances where such marks are subjected to a treatment such as a coating for reflection reducing purposes. Thus, it is difficult to optically detect the reflected images of the marks subjected to the reflection reducing treatment.
In this case, while it is conceivable to arrange a reflective surface in the mark transmission direction (the opposite direction to the illumination light source), in this case the light transmitted through around each mark is reflected by the reflective surface and thus a defocused negative mark image is formed on the mark. As a result, there is a problem that the blurred mark image is superposed around each mark and thus the mark position detection accuracy is deteriorated.
Then, in accordance with manufacturing processes a variety of masks including a mask subjected to a reflection reducing treatment, the ordinary mask, etc., are used. When effecting the position detection of these masks, there is a problem that the variation of the reflectance with respect to the position detecting illuminating light is as large as from 10 to 70%.